Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Havin growth from molten state
Reexamination Certificate
1998-03-27
2001-08-07
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Havin growth from molten state
C117S081000, C117S082000, C117S093000, C117S940000, C252S30140H
Reexamination Certificate
active
06270570
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fluoride crystal, an optical article for an excimer laser using the fluoride crystal, and a method of producing the fluoride crystal.
2. Description of the Related Art
The excimer laser has attracted attention as only one high output laser that can oscillate outside the ultraviolet region, so that the application thereof is expected in the electronics industry, chemical industry, and energy industry.
Specifically, the excimer laser is used in processing metals, resins, glass, ceramics and semiconductors, and in chemical reactions.
An apparatus for generating an excimer laser beam is known as an excimer laser oscillating apparatus. A laser gas filled in a chamber such as Ar, Kr, Xe, F
2
, Cl
2
is excited by electron beam radiation or electric discharge. The excited atoms bond with atoms that are in a ground state to produce a molecule existing only in an excited state. The produced molecule is called an “excimer”. Due to its instability, the excimer immediately discharges an ultraviolet ray and falls into the ground state. This phenomenon is called “bond free transition”. An apparatus for taking out a laser beam by amplifying the ultraviolet ray obtained by the transition in an optical resonator comprising a pair of mirrors is an excimer oscillating apparatus.
Among excimer laser beams, a KrF laser and an ArF laser produce light having a wavelength of 248 nm and light having a wavelength of 193 nm in a vacuum ultraviolet region, respectively. Therefore, a glass material having a high transmittance with respect to light having these wavelengths must be used in an optical system. Fluorites (i.e., calcium fluoride crystals) are preferable as the glass material for the optical system.
The method of producing a fluorite which has been conducted by the present inventors will be explained below.
FIGS. 9A
to
9
D are schematic views showing the steps of the method of producing a fluorite crystal which has been conducted by the present inventors.
In the step of
FIG. 9A
, a powdered raw material is placed in a container. In the step of
FIG. 9B
, the material placed in the container is melted and then cooled. In the step of
FIG. 9C
, solidified agglomerates are pulverized with a pulverizer made of stainless steel. In the step of
FIG. 9D
, a fluorite block is produced by melting and gradually cooling the pulverized agglomerates placed in a crucible for crystal growth.
The step of
FIG. 9B
is conducted for reducing the change of a bulk density before and after melting in the step of
FIG. 9D
, and further for removing impurities in the raw material. For obtaining a higher purity, these steps are repeated a plurality of times.
In the steps of
FIGS. 9B and 9D
, a scavenger which is a fluoride of a metal is added to the raw material in order to remove CaO generated by the reaction between the raw material (CaF
2
) and water or the like, or impurities originally existing in the raw material. For example, a ZnF
2
scavenger reacts with CaO to form CaF
2
and become ZnO and like, and oxygen is removed by removing the scavenger at the time of melting the raw material. As a result, CaO is removed as the impurity to obtain a fluoride crystal having an excellent transmittance characteristic.
The obtained fluorite block is cut in a desired thickness, processed and shaped to be a desired lens shape to be used as an optical article.
Although the conventional fluorite functions satisfactorily as an optical article for an ordinary optical system using a visual light, its optical characteristics deteriorated when radiation of a light beam having a short wavelength and a high output such as an excimer laser was repeated for a long time.
While seeking the cause of the deterioration, the present inventors found out that the cause was derived from not only impurities in the raw material but also the added scavenger. That is, with a large amount of the scavenger to be added for removing impurities such as oxygen, the scavenger itself and a reaction product thereof remain in the crystal of fluorite to deteriorate the internal transmittance and durability of fluorite. However, oxygen cannot be sufficiently removed by merely reducing the amount of the scavenger.
SUMMARY OF THE INVENTION
The present invention has been completed based on the above finding. A main object of the present invention is to provide a fluoride crystal having a high transmittance with respect to a short wavelength light, without the risk of deterioration of the transmittance characteristics even when a high output of short wavelength light is repeatedly radiated thereto for a long time.
Another object of the present invention is to provide a fluoride crystal preferable as an optical article for an excimer laser of a large aperture (diameter of 250 mm or more).
Yet another object of the present invention is to provide a fluoride crystal to serve as a highly reliable optical article.
Yet another object of the present invention is to provide a method of producing a fluoride crystal having a high transmittance, without the risk of deterioration of the transmittance characteristics even when a short wavelength light having a high output is repeatedly radiated thereto for a long time.
Yet another object of the present invention is to provide an optical article for an excimer laser without the risk of deterioration of the transmittance characteristics even when a high output of short wavelength light is repeatedly radiated thereto for a long time.
In order to solve the above-mentioned problems, the present inventions as described below are provided.
A first aspect of the present invention is a fluoride crystal containing at least one kind of atom selected from the group consisting of Zn, Cd, Pb, Li, Bi and Na with a content of 10 ppm or less, and having an internal transmittance of 70% or more with respect to a 135 nm wavelength light.
A second aspect of the present invention is a fluoride crystal containing at least one kind of atom selected from the group consisting of Zn, Cd, Pb, Li, Bi and Na with a content of 10 ppm or less, and containing oxygen with a content of 50 ppm or less.
A third aspect of the present invention is the above fluoride crystal further having the internal transmittance of 70% or more with respect to 135 nm wavelength light.
A fourth aspect of the present invention is a method of producing a fluoride crystal comprising a refining step of adding a scavenger to a raw material of calcium fluoride for at least once, and a crystal growth step of further adding the scavenger to the refined material and growing a crystal by using a crucible lowering method, wherein the amount of the scavenger to be added in the first refining step is 0.04 to 0.1 mol % based on the raw material, and wherein the total amount of the scavenger to be added in the subsequent refining step and the crystal growth step is 10% to 50% based on the amount of the scavenger to be added in the first refining step.
A fifth aspect of the present invention is the above method of producing a fluoride crystal, wherein the oxygen content of the scavenger is 0.1% by weight or less.
A sixth aspect of the present invention is the above method of producing a fluoride crystal, wherein the scavenger is zinc fluoride.
A seventh aspect of the present invention is an optical article for an excimer laser comprising the above fluoride crystal.
An eighth aspect of the present invention is an optical article for an excimer laser comprising the fluoride crystal produced by the above method of producing a fluoride crystal.
A ninth aspect of the present invention is an exposing apparatus comprising an illuminating light source, a stage for mounting an object to be exposed, and a plurality of optical articles provided in an optical illumination system and/or an optical projection system, wherein at least one of the optical articles comprises a fluoride crystal having an internal transmittance of 70% or more with respect to a 135 nm wavelength light, and containing at least one kind of at
Ichizaki Toshio
Ohba Tomoru
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Kunemund Robert
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